There are a number of information-related services today, and there is a need of preventing unauthorized uses of such services by a third person or an unauthorized user. To do this, there has been a technology of distributing, only to an authorized user, an encryption key needed for deciphering (decrypting) encrypted text (encrypted data). Using this technology, a user is provided with services via a communication and/or a broadcast communication path. Such services can be: for example, a service of distributing movies or the like contents or a service of providing a network resource such as wireless LAN.
Meanwhile, users are demanding for time-limited services which provide information only during a predetermined period designated: i.e., during a particular period specified. Such services are for example: a paid TV broadcasting which can be viewed for only 1 hour, or a wireless LAN service which can be used for only 30 min.
In response to the demand, a format of an encryption key for realizing a time-specified information service is disclosed in the following documents: Non-Patent Citation 1 (M. Kuribayashi and H. Tanaka: “A New Key Generation Method for Broadcasting System with Expiration Date,” 27th Symp. on Information Theory and Its Applications (SITA2004), pp. 323-326 (2004)); and Non-Patent Citation 2 (Suga, Iwamura: “Key Generation Method for Access Control for One Dimensional Contents”, Computer Security Symposium 2004 (css2004), pp. 481-486 (2004)). Non-Patent Citation 1 discloses a method of following updating of key during a limited period of time, by utilizing a behavior of logistic mapping. Non-Patent Citation 2 discloses a method of deriving an encryption key which allows decryption of a specific portion of contents.